Retrospectively casting an eye over an incredible year for both Apple and its customers, one of the most surprising developments of 2010 was the Mac’s long-overdue maturity into a serious gaming platform after years of false hopes and promises.

More surprising than even that, though, is the fact Apple almost had nothing to do with it: even while Cupertino oiled and massaged iOS into a platform capable of rattling the nerves of gaming’s most unassailable colossus, they continued to ignore Mac gamers and its developers.

So who was responsible for the Mac Gaming Renaissance of 2010? There’s no one company in particular, but let’s start with Valve.

Valve’s excellent Steam for Mac digital delivery service is having a Halloween sale this weekend, pretty much guaranteeing that if it’s spooky, on the Mac and features zombies, ghosts or monsters, you’ll be able to download it for the price of a Monster Mash.

If you’re a Mac gamer, here are the titles that are available at a discount this weekend:

Earlier this month, Valve finally brought their fantastic team-based cooperative zombie shooter Left 4 Dead 2 to the Mac. It was a much-appreciated port, but it was a bit odd, in that Valve had opted to bring Left 4 Dead 2 to the Mac before the original game in the series, which was built on the same engine.

According to Valve, the delay in bringing Left 4 Dead to the Mac simply had to do with the first game in the series being a bit more complicated to port to OS X than they had anticipated. They promised a release by Halloween, though, and I’m delighted to say that they’ve been as good as their word: load up Steam for Mac and you can now download Left 4 Dead.

The best news is it’s dirt cheap: Left 4 Dead will only cost you $9.99 if you buy it this week. Why not spend an extra 5 bucks, though, and pick up both Left 4 Dead and Left 4 Dead 2 for $14.99? That’s just a steal.

Like Valve’s previous games, Left 4 Dead is Steam Play compatible, which means once you own it on the Mac, you also own it on the PC, and vice versa. If you intend on shooting zombies with some buddies this Halloween, though, make sure you have OS X 10.6.4 installed, as well as a 2GHz dual-core Intel CPU, an ATI Radeon x2400 or NVIDIA 8600M GPU or better.

Valve Software is a company which has been heaping much love on Mac gamers over the last few months, and whose much anticipated physics-based first person puzzler Portal 2 will be debuting early next year on both PC and Mac simultaneously. It’s about to get even better: they’ve just announced their next game, Dawn of the Ancients 2, coming sometime in 2011. Even better? It’s coming to the Mac.

Although Valve released their other Source engine titles with astonishing rapidity when they finally brought Steam to Mac earlier this year, but one sad lingering hold out in the games library disgorgement was the team-based zombie-shooter series, Left 4 Dead, with the delay largely chalked up to some bugs in OS X’s graphics drivers that Valve helped Apple iron out.

We’d previously heard tell that now that those graphic issues are fixed, Valve as hard at work to bring Left 4 Dead and Left 4 Dead 2 to OS X by October… and now, if a casual mention over at Macworld is anything to go by, it looks like that date might have been further locked down to October 5th, along with the latest Left 4 Dead and Left 4 Dead 2 add-on pack, “The Sacrifice.”

Looks like for us Mac gamers, the Halloween season is going to be filled with even more zombie mutilating than usual. Expect Cult of Mac to organize a few games of Left 4 Dead once it finally hits our platform.

Valve Software’s transcendent physics and teleportation puzzler Portal already runs sublimely through Steam for Mac… as long as you have a modern MacBook Pro or iMac. But what if Portal wasn’t a modern game, but a classic Macintosh text adventure? This brilliant mock ad re-imagines Portal as a lost game rediscovered through a lost Saturday morning advertisement, complete with an appearance by a badly puppeted GlaDOS. Brilliant… although that green text really looks more at place on an Apple II, don’t you think?